1. Field of the Invention
The present invention relates to a method of protection against corrosion of reinforcing bars, prestressing cables and similar articles located within structures.
Structures which are fabricated from concrete, such as piers, harbor walls, offshore platforms, prestressed concrete located in salt water, concrete having additives or aggregates containing chloride ions, are subject to damage due to corrosion.
Corrosion of reinforcing bars may occur specifically in concrete structures which
are located in salt water (ocean) (offshore structures) PA1 are manufactured with sand which contains salt, PA1 are manufactured with sand stemming from the ocean PA1 are located on beaches PA1 are treated with thawing salts PA1 are located in industrial areas (acid exhaust gases), or PA1 are manufactured with quick setting additives or antifreeze agents containing chloride ions. PA1 A coating of the reinforcing bars by synthetic resins, specifically epoxides, leads to a complete protection against corrosion in case that the coating is sufficiently thick and will not be damaged during the placing or mounting, respectively. The drawbacks are: high costs, no adhering of concrete to the protective layer, electrical insulation, and at damaged areas the generation of local cells which lead to a locally accelerated corrosion. PA1 Galvanized usually zinc plated iron or steel bars are excellently protected against corrosion. It is, however, known that the zinc coating will be dissolved due to the alkalinity of the cement. Therefore, the protective effect is limited regarding durability. PA1 A third known method is cathodic corrosion protection. It consists of applying a predetermined voltage which inhibits the corrosion of iron. The drawbacks of this method are, however, that at different areas voltages should be applied depending on the local concentration of salt and voltage drop within the concrete. Practically such a method is very difficult to be controlled in a reinforced concrete structure. An unsuitably applied cathodic protection against corrosion may lead to the loss of the adherence of the reinforcement to the concrete or even to an accelerated corrosion. PA1 10-100% of alkali metal nitrite or alkaline earth metal nitrite PA1 0-90% of dicarboxylic acid PA1 0-90% of amine.
Corrosion is an electrochemical process. The reaction products, rust and complex compounds with chloride, have a larger volume than the original starting materials. This leads to the formation of local cracks and spalling; therefore, additional salt can penetrate and thus the course of corrosion will accelerate.
Certain specific chemicals can prevent or delay such a process. These chemicals are added to the concrete in the form of an admixture during the production thereof and supposedly prolongs the durability of concrete structures. Such additives are marketed and sold already in the U.S.A. and in Japan. Generally, their main or sole content is calcium nitrite. Furthermore, a large number of patent specifications or published applications exist regarding this matter, such as the Canadian Pat. No. 802 281, U.S. Pat. No. 3,427,175, W. German Offenlegungsschift No. 3 005 896, Japanese Pat. Publication 33 940, U.S. Pat. No. 3,210,207, and U.S. Pat. No. 3,801,338. Due to their limited efficiency other inhibitors such as benzoates, chromates, phosphates, amines, etc. have hitherto not found a large technical application.
The drawback of the presently available corrosion inhibiting additives for concrete is that they are distributed throughout the concrete which leads to extremely high dosages of such additives. Due to their high toxicity, highly concentrated nitrite solutions or solid nitrite additives are not welcome at construction sites. It is a known fact that the effectiveness of such additives regarding the inhibition of corrosion is related to their dosage and simultaneously to the concentration of salt in the concrete structure. The case of the formation of large cracks due to an improper concreting a reinforcing bar which is located at such area will not be protected against corrosion in spite of the presence of corrosion inhibiting additive in the concrete because the salt at such an area may be present in a high concentration.
Due to its highly alkaline medium (pH&gt;12) concrete possesses corrosion protective features because reinforcing bars are passivated in this pH range. In case of a high addition of latent hydraulic materials the pH value may, however, decrease to a range which is no longer corrosion protective. However, concrete is seldom so dense that no chloride penetration occurs. Concrete may contain from the outset chlorides from additives, water or additive materials so that concrete may hardly protect iron against corrosion over an extended time span.
2. Description of the Prior Art
Presently, the following three methods are practically followed: